The present invention relates to a fullcolor LED display system displaying gradation-rich, multicolor images by combining, for example, LED lamps of three primary colors of RGB (red, green and blue). Particularly, the invention relates to a system of pulse-width modulation method for lighting and activating an LED lamp by an activating pulse having been pulse-width modulated based on gradation data for each color.
Following the development of high-luminance blue LEDs (light-emitting diodes), fullcolor LED display systems, combining the three primary colors RGB, are beginning to become popular. An example of a specification of a typical device is as below. A display screen is in a large size of 2.4 meters in height and 3.4 meters in width. A total of 61,440 pixel lamps of 480 lines vertically and 128 dots laterally are arrayed in this screen. Each of the pixel lamps is an LED-multicolor-assembled lamp in which respective LEDs in the three primary colors RGB are densely gathered. Pixel data for driving one pixel consists of a total of 24 bits, that is 8 bits respectively for each RGB. The displaying gradation for each of the colors RGB is 256 tones respectively, and thus, a fullcolor expression of 16,777,216 colors is made possible.
In this type of fullcolor LED display system, it is possible to use, as its video source, an NTSC video signal used in a regular television broadcasting system or a VTR. An NTSC video signal having been input to a display-control device is A/D converted, and is converted and processed into digital signals of a total of 24 bits of 8 bits respectively for RGB. Image data for one screen, containing (61,440xc3x9724) bits corresponding to the 61,440 pixel lamps, is buffered in a frame memory. From this frame memory, image data of 24 bits for a single pixel is respectively distributed to a activating circuit of each pixel lamp, and is latched to a register in the activating circuit.
In the pixel-lamp activating circuit, the red LEDs are activated and lit at a tone corresponding to the 8 bits of red data latched in the register. Similarly, the green LEDs are activated and lit at a tone corresponding to the 8 bits of green data, and the blue LEDs are activated and lit at a tone corresponding to the 8 bits of blue data.
Such a gradation control is generally conducted by a known pulse-width modulation method. A clock pulse of a sufficiently-high constant frequency is continuously generated; a (28)=8-bit counter is incremented by the clock pulse; and an 8-bit count value of the counter is repetitively changed at a constant period Ts from all xe2x80x9c0xe2x80x9d to all xe2x80x9c1xe2x80x9d. By comparing, with a digital comparator, the magnitude between this 8-bit calculated value and the 8-bit gradation data latched in the register of the activating circuit, an activating pulse with a pulse width Tw corresponding to the 8-bit gradation data and with the above-mentioned period Ts is output from the comparator. The pixel-lamp activating circuit feeds a constant current through the LED and lights it for a time period of the pulse width Tw of the activating pulse. This pulse lighting is repeated at period Ts.
That is, the pulse width Tw of the activating pulse with a period Ts is determined proportional to the binary value of the 8-bit gradation data, and a displaying luminance corresponding to the 8-bit gradation data is obtained by pulse-lighting the LED with a constant current for time Tw during period Ts.
Even nowadays, the mainstream television-image display devices are CRT television sets. Since the RGB three-colored fluorescent materials of the CRT television sets do not illuminate in proportion to the voltage of the input video signal, the relation between the input signal and the optical output is nonlinear. As well known, such a characteristic is referred to as GAMMA. If the nonlinearity (gamma) of the CRT is corrected at each television set, the television set becomes complicated and expensive. Thus, in the current television method, signals having been gamma-corrected at the sending side are broadcasted. The actual gamma value becomes a quite different value according to measuring conditions and measuring methods. In the NTSC method, gamma correction is conducted assuming that the gamma value of the image-display device is 2.2.
However, in an LED display system, the relation between the input signal and the optical output is approximately linear, and is not nonlinear as of a gamma of a CRT television set. The relation is not completely nonlinear, but the characteristic is significantly different from the gamma of a CRT.
If a gamma-corrected NTSC video signal is taken as a video source of an LED display system, it would be necessary to carry out an inverse-gamma correction with means of some kind and carry out gradation control according to the approximately-linear characteristic of the LED, if a high-quality image displaying were to be realized.
In a Japanese Patent Application Laid-open Publication (No. 7-306659) issued in 1995, a technique as follows was disclosed concerning a multicolor LED display unit:
(1) An LED display unit (screen) is formed by orderly arraying a multitude of LEDs in the three primary colors RGB. An LED lighting circuit for lighting each of the LEDs and adjusting the lighting color and brightness thereof is installed to the unit.
(2) The LED lighting circuit comprises: a pulse-width modulation circuit which outputs an activating pulse corresponding to an inputted gradation data; and an LED activating circuit which lights the LED with the activating pulse from the pulse-width modulation circuit.
(3) The pulse-width modulation circuit comprises: a nonlinear counter in which the relation between time and a count value takes a nonlinear action; and a digital comparator which compares the magnitude between the count value of the nonlinear counter and the gradation data stored to a buffer memory to generate the aforementioned activating pulse.
(4) The nonlinear counter comprises: a pulse generator which generates a count pulse of 16 types, each having a different period; a selection circuit which selects one type of count pulse out of the 16 types; a binary counter which counts the count pulse having been selected by the aforementioned circuit; and a decoder circuit which generates a selection signal for selecting the 16 types of count pulses from the higher-order 4 bits of the binary counter.
(5) When the count value of the binary counter is small, the selection circuit has selected a count pulse having a short period according to the selection signal from the decoder circuit, and thus, the count value of the binary counter increases rapidly. When the count value of the binary counter becomes large, the selection signal from the decoder circuit changes, and the selection circuit selects a count pulse having a long period, and thus, the count value of the binary counter increases slowly.
(6) Gradation data is successively sent from an external device, such as a display controller, to the LED display system, and is temporarily stored in a memory. The gradation data stored in the memory is input to the digital comparator via the buffer memory. The pulse width Tw of the activating pulse which is output from the digital comparator is nonlinearly modulated in view of the gradation data; in a range where the gradation data is small, the rate of change of the pulse width Tw is small, and as the gradation data becomes large, the rate of change of the pulse width Tw becomes large.
In the conventional multicolor LED display unit as described-above, by adopting gradation control according to nonlinear-pulse-width modulation, in the case where a gamma-corrected NTSC video signal is taken as a video source, it is possible to carry out an inverse-gamma correction of a line-graph like approximation which matches the approximately linear characteristic of the LED, to carry out image displaying of a higher quality.
However, in this known technique, since an inverse-gamma correction of a line-graph like approximation is conducted, it is difficult to carry out an inverse-gamma correction of high quality with a simple circuit structure, and it is also difficult to realize a superior image quality of sufficient satisfaction. Further, since a circuit structure, which carries out gradation control by nonlinear pulse-width modification, is installed to the LED display unit, there were structural problems as described below when considering adaptation to an embodiment of particularly a large-screen LED display device.
In a downtown area of a city, there are seen many large-screen full-color LED displays installed on walls of buildings. In such a system, a configuration, wherein screen modules installed on such as a building wall is connected with data-sending modules arranged within a building room through data-transmission cables, is adopted. A screen module is equivalent to a required number of the LED display units of the aforementioned known document being connected together. A data-sending module is equivalent to what is represented as the external device such as the display controller in the aforementioned known document.
In the full-color LED display system as described above, it is desired to enhance image quality by optimizing a display-gradation-control characteristic through various factors, such as variably controlling, in a suitable manner, control characteristics of display tones according to gradation-expression characteristics (gamma-correction characteristic of a TV signal is one such characteristic) of an image data to be displayed, or, variably controlling, in a suitable manner, the control characteristics of display tones according to if it is daytime when sunlight is shining or nighttime when it is not.
In order to realize the aforementioned function, an optimization information for the display-gradation-control characteristic would be sent from the data-sending module (a computer for controlling display) which feeds image data to the screen module. In the known technique, the characteristic of the nonlinear counter, which is installed to the LED display unit (the structural component of the screen module), would be successively changed by a signal fed from the display controller (the data-sending module).
It is possible to realize such a circuit system. However, matters, such as what kind of signal is to be fed from the data-sending module to which part of the nonlinear counter in the multitude of LED display units structuring the screen module and how its characteristic is to be variably controlled, were not the theme of the invention disclosed in the aforementioned known document.
In the aforementioned known document, it is described that the pulse generator (generating the 16 types of count pulses), which is a structural component of the nonlinear counter, may be a program counter, and that its set value (a value for determining the respective periods of the 16 types of count pulses) can be optimized from an external point. From this description, it is possible to think of a control system which changes the set value of the pulse generator within the nonlinear counter in the multitude of LED display units structuring the screen module by signals from the data-sending module connected to the screen module through the data-transmission cable. However, in such a case, the control system would have a complicated and expensive circuit structure requiring a multitude of signal-sending lines. Even when adopting such a complicated and expensive circuit structure, it is only possible to carry out gradation control of the aforementioned line-graph-like characteristics, and to carry out an extremely limited characteristic change of modifying the slope of each of the line segments of the line graph.
A control system apart from the aforementioned type is to be considered. For example, in the aforementioned known technique, it is possible to think of a system configuration wherein: the pulse generator, which is a structural component of the nonlinear counter, is installed to the side of the data-sending module; and the count pulses of 16 kinds which are output from the pulse generator are transferred to the screen module through the data-transmission cable and are input to the selection circuit in the nonlinear counter. Then, in order to change the characteristic of the nonlinear counter, the characteristic of the pulse generator is variably set by the computer of the data-sending module, and the period of the 16 types of the count pulses is appropriately modified. However, alike the aforementioned system, this control system becomes a complicated and expensive circuit structure. Even when such a complicated and expensive circuit structure is adopted, it is only possible to carry out gradation control of the aforementioned line-graph-like characteristics, and to carry out an extremely limited characteristic change of modifying the slope of each of the line segments of the line graph.
An object of the present invention is to provide a system configuration which, in accordance to a gradation-expression characteristic of such as an NTSC video signal to be taken as a video source, can easily carry out suitable correction of such characteristic to adapt to the characteristic of an LED by means of a simple circuit system, and can carry out full-color image display of high quality, in a full-color LED display system which is system-configured from a screen module and a data-sending module.
A fullcolor LED display system according to the first invention is specified by the following matters (11)-(17), wherein:
(11) the above comprises a screen module for displaying a multicolor image on a screen in which a multitude of first-color LEDs, second-color LEDs and third-color LEDs are orderly arrayed; and a data-sending module which gives a control signal and image data to be displayed on the screen module;
(12) the screen module and the data-sending module are connected by data-sending means;
(13) the image data is an assembly of gradation data for each colors of each pixels on the screen; and on the screen module, for each pixel on the screen, there are installed first-color gradation-control circuits, second-color gradation-control circuits and third-color gradation-control circuits for pulse-lighting the LEDs;
(14) the gradation-control circuit for each color comprises: an n-bit counter for counting high-speed pulse trains given from the data-sending module; a register for latching the gradation data given from the data-sending module; a digital comparator for comparing magnitude between an n-bit count value from the n-bit counter and the gradation data latched to the register; and a constant-current driver for turning ON and OFF a current-passing to the LED according to a binary output of the digital comparator;
(15) the data-sending module comprises: a frame memory for temporarily storing image data to be displayed on the screen module; an image-data-transfer-control means for reading out the image data from the frame memory, and for outputting, to the data-sending means, the image data along with a predetermined data-transfer clock in a predetermined pixel order; first-color high-speed pulse-train generating means, second-color high-speed pulse-train generating means, and third-color high-speed pulse-train generating means for generating high-speed pulse trains to be given to the respective first-color gradation-control circuit, second-color gradation-control circuit a nd third-color gradation-control circuit; and a high-speed pulse-train outputting means for outputting, to the data-sending means, the respective high-speed pulse trains for the respective first color, second color and third color;
(16) the data-sending means and the screen module comprise: a data-transfer-control system for latching the respective gradation data of each color of each pixel, having been outputted from the data-sending module, to the register in the gradation-control circuit for the corresponding color and the corresponding pixel; and a signal-transfer system for applying the first-color high-speed pulse trains, the second-color high-speed pulse trains and the third-color high-speed pulse trains, having been outputted from the data-sending module, as a count input to the n-bit counter in the gradation-control circuit of the corresponding color; and
(17) the high-speed pulse-train generating means for each color repetitively generate, with a constant period, high-speed pulse trains of (2n) pieces or a number closely therebelow, of which pulse intervals vary with time according to a varying characteristic having been set.
A fullcolor LED display system according to the second invention is characterized in that:
the data-sending module comprises a single-system high-speed pulse-train generating system which is shared among process systems for the first color, second color and third color; and
the data-sending means and the screen module comprise a signal-transfer system for applying the high-speed pulse trains of a single system, having been outputted from the data-sending module, as a count input of the n-bit counter in the gradation-control circuit of each color.
A fullcolor LED display system according to the third invention is specified by the following matters (21)-(28), wherein:
(21) the above comprises a screen module for displaying a multicolor image on a screen in which a multitude of first-color LEDs, second-color LEDs and third-color LEDs are orderly arrayed; and a data-sending module which gives a control signal and image data to be displayed on the screen module;
(22) the screen module and the data-sending module are connected by data-sending means;
(23) one pixel is formed of the first-color LED(s), the second-color LED(S) and the third-color LED(s) adjacently arranged on the screen; and
in the screen module there is installed: one gradation-control circuit for pulse-lighting the first-color LED(s), the second-color LED(S) and the third-color LED(s) forming the same pixel; and a color-select circuit for selecting the first-color LED(s), the second-color LED(s) and the third-color LED(s) forming the same pixel;
(24) the image data is an assembly of gradation data for each color of each pixels on the screen;
one period for lighting and activating the LEDs according to the image data is divided into three of: a first-color activating period for lighting and activating the first-color LED(s) according to first-color gradation data; a second-color activating period for lighting and activating the second-color LED(s) according to second-color gradation data; and a third-color activating period for lighting and activating the third-color LED(S) according to third-color gradation data;
divided-time intervals of the first-color activating period, the second-color activating period and the third-color activating period are set to be a short time to an extent in which human sight cannot recognize that the three colors are lighted with a time difference;
(25) the gradation-control circuit comprises: an n-bit counter for counting high-speed pulse trains given from the data-sending module; a register for latching the gradation data given from the data-sending module; a digital comparator for comparing magnitude between an n-bit count value from the n-bit counter and the gradation data latched to the register; and a constant-current driver for turning ON and OFF a current-passing to the LED according to a binary output of the digital comparator; and
first-color LED(s), second-color LED(s) and third-color LED(s) of the same pixel are connected in parallel to the constant-current driver via the color-select circuit;
(26) the data-sending module comprises: a frame memory for temporarily storing image data to be displayed on the screen module; an image-data-transfer-control means for reading out the image data from the frame memory, and for outputting, to the data-sending means, the image data along with a predetermined data-transfer clock in a predetermined pixel order; high-speed pulse-train generating means for generating high-speed pulse trains to be given to the gradation-control circuit; and means for outputting, to the data-sending means, the high-speed pulse trains;
(27) the high-speed pulse-train generating means orderly generates, with a constant period, high-speed pulse trains of (2n) pieces or a number closely therebelow, of which pulse intervals vary with time according to a varying characteristic having been set according to color in the respective first-color activating period, the second-color activating period and the third-color activating period; and repeats this; and
(28) the data-sending module carries out, by giving predetermined data to the screen module via the data-sending means: a first-color activating process for extracting, from the image data in the frame memory, the first-color gradation data for each pixel, distributing the gradation data to the gradation-control circuit of each pixel, and activating the first-color LED(s) of each pixel for a predetermined time in a unison; a second-color activating process for extracting, from the image data in the frame memory, the second-color gradation data for each pixel, distributing the gradation data to the gradation-control circuit of each pixel, and activating the second-color LED(s) of each pixel for a predetermined time in a unison; and a third-color activating process for extracting, from the image data in the frame memory, the third-color gradation data for each pixel, distributing the gradation data to the gradation-control circuit of each pixel, and activating the third-color LED(s) of each pixel for a predetermined time in a unison.
A fullcolor LED display system according to the fourth invention is characterized in that the high-speed pulse-train generating means in the data-sending module comprises: a waveform memory having stored therein digital data in which the pulse trains are expressed as a static binary waveform pattern; and a memory-data-reading means for repetitively generating, with a constant period, high-speed pulse trains of (2n) pieces or a number closely therebelow, wherein pulse intervals vary with time according to the varying characteristic having been set, by read-accessing the waveform memory at a predetermined speed and in a predetermined order, and outputting, in series, digital data of the binary waveform pattern.
A fullcolor LED display system according to the fifth invention is characterized in that the data-sending module comprises a characteristic-varying means for changing the varying characteristic of the high-speed pulse trains by rewriting the data in the waveform memory.
A fullcolor LED display system according to the sixth invention is characterized in that the high-speed pulse-train generating means in the data-sending module comprises a function-arithmetic-operation means for repetitively generating, with a constant period, the high-speed pulse trains by conducting, at high speed, a function-arithmetic operation according to a program in which a time, until a succeeding pulse Pi+1 is output after a pulse Pi has been output, is expressed as a function of i.
A fullcolor LED display system according to the seventh invention is characterized in that the data-sending module comprises a characteristic-varying means for changing the varying characteristic of the high-speed pulse trains by changing the function having been programmed to the function-arithmetic-operation means.
A fullcolor LED display system according to the eighth invention is characterized in that the data-sending module has a plurality of characteristic information, which defines the varying characteristic of the high-speed pulse trains, having been preset thereto; and that the characteristic-varying means includes a characteristic-switching means for selectively adopting the characteristic information having been preset.
A fullcolor LED display system according to the ninth invention is characterized in that the data-sending module comprises: an analyzing means for carrying out an analysis, according to an appropriate algorithm, a gradation-expression characteristic of image data to be displayed on the screen module; and a changing means for appropriately changing the varying characteristic of the high-speed pulse trains, by the characteristic-varying means, according to a result of the analysis.
A fullcolor LED display system according to the tenth invention is characterized in that the data-sending module comprises a changing means for appropriately changing the varying characteristic of the high-speed pulse trains, by the characteristic-varying means, according to a predetermined control information attached to image data to be displayed on the screen module.
A fullcolor LED display system according to the eleventh invention is characterized in that the data-sending module comprises a changing means which obtains information related to a condition of light ray to which the screen module is subjected, and which appropriately changes the varying characteristic of the high-speed pulse trains, by the characteristic-varying means, according to the information.
A fullcolor LED display system according to the twelfth invention is characterized in that the data-sending module comprises a changing means which obtains information related such as to season, time of day, and climate, and which appropriately changes the varying characteristic of the high-speed pulse trains, by the characteristic-varying means, according to the information.
A fullcolor LED display system according to the thirteenth invention is characterized in that, as for a group of the LEDs with the same color in a plurality of pixels adjacently arranged on the screen, a group of the gradation-control circuits for the respective LEDs is integrated into one integrated circuit; and in the group of gradation-control circuits, one n-bit counter is shared among the respective gradation-control circuits.